1. Field of the Invention
This invention relates to the field of water-leachable pre-emergence herbicidal compositions which are characterized by controlled release of the herbicide contained in the composition on exposure to moisture.
2. Description of the Prior Art
Herbicides are substances used to destroy plants, especially weeds, or to check their growth. Commonly, for weed control in agriculture, water soluble herbicides are employed in the soil to prevent the emergence of undesired plants, e.g. weeds, which compete with desired plants. Such herbicides are referred to as pre-emergence herbicides and are usually dependent on rain or irrigation to wash the herbicide into the soil. Due to variable rainfall, the efficiency of washing such herbicides into the soil poses a difficult problem in that with increased rainfall the extent of penetration into the soil layer by the herbicide may be excessive with highly soluble herbicides, i.e., the herbicide is leached to a zone below that in which the undesired, i.e., weed seeds are germinating. The result is an unsatisfactory weed control. In addition to the solubility problem, penetration differentials are also attributable to the type of soil, i.e., sandy, loose soil versus muck soil, and importantly, the relative ability of the soil components to absorb the herbicide. It is apparent that the success of weed control among agricultural crops such as beans and tomatoes with pre-emergence herbicides is a complex problem which must entail consideration of the soil, crop, weed and environmental conditions, especially rainfall.
In addition to the problem of leaching the herbicide to below the zone of weed seed germination, is the problem generated by excessively long residence time of the herbicide in the upper zones of the soil where, on a protracted basis, adverse effects of the herbicide can occur with respect to the germination of desired seeds, or alternatively, with respect to established plants or trees. Thus, the leachability of the herbicide must permit adequate penetration to the soil zone where weed seeds are germinating but at the same time should be sufficient to remove the herbicide from the growth zone within reasonable periods of time. Ideally, the herbicide should have a leachability which would permit effective presence only during the germination period of the undesired seeds, after which the herbicide is effectively removed from the growth zones of the soil.
Amiben is a known pre-emergence herbicide which is of high water solubility. This commercial product, however, suffers from the aforesaid difficulties when employed in the soil, primarily because of water-solubility, i.e., heavy rain may leach the herbicide below the zone of weed seed germination. Amiben also is not as readily absorbed by the soil as are other pre-emergence herbicides, in which event the leaching rate of Amiben in sandy soil and clay loam soil is substantially the same, as evidenced by the relatively small changes in leaching rates required to obtain similar weed control in both sandy and clay loam soil. Even in muck soil, Amiben appears to move rather readily through the soil profile. Other herbicides, such as Diuron CIPC and PCP require several fold changes in rate to produce similar herbicidal effect with broad changes in the soil types. It is suggested the Amiben, when absorbed, is more strongly absorbed than other herbicides, but it remains that Amiben is not absorbed to any appreciable degree by soil constituents.
Attempts to effect control of leaching of Amiben have taken several approaches. Salt formation with metals such as barium, aluminum and iron did not appreciably vary field performance, presumably because the salts reach an equilibrium with naturally occurring cations in the soil. Since Amiben is a substituted benzoic acid, amides and esters were prepared and found to have major differences in leaching characteristics. Such derivatives, however, require water to hydrolyze the derivative for effectiveness.
In U.S. Pat. No. 3,560,196, it is proposed to combine Amiben, inter alia, with a granular carrier coated with a silicate binder which binder is treated to render it partically insoluble. The rate of release of the biologically active material is principally dependent on the silicate binder, more specifically the relative insolubility of the silicate binder.
U.S. Pat. No. 3,336,155 describes the coating of solid and liquid particles with polymers separated from an organic solvent solution thereof by the addition, under stirring, of an organic liquid which is a non-solvent for the polymer, but is miscible with the polymer solvent, and the solid or liquid particles which must be insoluble in the miscible solvent and nonsolvents. Among the numerous polymers and materials to be coated recited in U.S. Pat. No. 3,336,155 are cellulose acetate and the water soluble herbicide, sodium (2,4-dichlorophenoxy) acetate.
U.S. Pat. No. 3,318,769 describes a thermoplastic resin such as cellulose acetate having dissolved therein up to about 70% by weight of a dialkyl beta-halogen substituted vinyl phosphate insecticide. These water insoluble insecticides evaporate into the surrounding atmosphere over an extended period.
The benzoic acid pre-emergence herbicides are well known compounds and have been described in the patent literature (See U.S. Pat. Nos. 3,013,873 and 3,014,063).
Amiben is particularly characteristic of the benzoic acid-herbicides which are especially affected by water solubility and, consequently, the problem of excessive leaching by water, e.g. rainfall, in the soil. The known class of benzoic acid herbicides includes mono-, di-and tri-substituted benzoic acids, in which the substituents are halo, amino, nitro and lower alkoxy. The most effective of such derivatives are the tri-substituted benzoic acid herbicides, particularly those containing chloro substituents, most preferably at least two chloro substituents. Thus, Amiben is 3-amino-2,5-dichlorobenzoic acid (U.S. Pat. No. 3,013,873); Banvel, another commercial herbicide is 2-methoxy-3,6-dichlorobenzoic acid. Other such herbicides include, for example, trichlorobenzoic acid; 3-nitro-2,5-dichlorobenzoic acid (Dinoben) and similar such tri-substituted benzoic acids.